1. Field of the Invention
This invention relates to a high-dielectric constant ceramic composite, and more particularly to a high-dielectric constant ceramic composite that can be satisfactorily employed as a dielectric material in a multilayer ceramic capacitor.
2. Description of the Prior Art
A dielectric material for use in ceramic capacitors is required to have electrical properties such as a high dielectric constant, a low temperature coefficient in the dielectric constant, and a small dielectric loss. Further, the required electrical properties include a dielectric constant which is less dependent on a biased electric field, and a large value of the product of its capacitance and the resistance, known as the CR value. To evaluate the reliability of a dielectric material, reliability tests such as a high-temperature load test, a high-humidity load test, and a heat-cycle test are performed.
In the process of manufacturing multilayer ceramic capacitors, the electrode materials and dielectric materials thereof are fired simultaneously. Thus, the firing temperature of the dielectric materials is required to be lower than 1100.degree. C. This low firing temperature allows the use of Ag, which are less costly than Pt, Pd and the like, as the electrode material. For this reason, dielectric materials of the BaTiO.sub.3 group have been widely used. However, in recent years, dielectric materials other than BaTiO.sub.3, e.g., lead-based dielectric materials, which are called relaxors and represented by a general expression of Pb(B.sub.1, B.sub.2)O.sub.3, have become of major interest. The lead-based dielectric materials serve o lower the firing temperature required for the process of manufacturing dielectrics of multilayer ceramic capacitors.
Further, the materials serve to improve the electrical an mechanical properties of the dielectrics employed in the capacitors. For this reason, there have been disclosed several relaxor, such as the lead iron niobate group disclosed in Japanese Laid Open Patent (J. L. O. P.) No. 57-57204, the magnesium-lead niobate group disclosed in J. L. O. P. No. 55-51758, and the lead magnesium tungstate group disclosed in J. L. 0. P. No. 52-21699. Further, the inventors of the present invention developed and disclosed the zinc-lead niobate group in J. L. 0. P.s No. 61-101460 and No. 61-155245.
As described above, various sorts of relaxor have been developed. However, the durability thereof under severe environmental conditions e.g. under a high-temperature load test) has not been sufficient. Moreover, when these relaxor are employed in the production of multilayer ceramic capacitors, the product of the capacitance and resistance is reduced during the high-temperature load test.
The inventors of this invention have disclosed a combined material of BaTiO.sub.3 and Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 as a high dielectric constant ceramic composite. The composite can be fired at a low temperature and has a low temperature coefficient of the dielectric constant. This material has been disclosed in Japanese Patent Application No. 63-156062.
In the view of the high-humidity load rest results alone, another improved composite has been disclosed in Japanese Patent Application No. 62-254310. Specifically, glass of Bi.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -Ag.sub.2 O was added to a composite of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Mg.sub.1/3 -Nb.sub.2/3)O.sub.3 -PbTiO.sub.3. However, in this disclosure the heat-cycle test was not taken into consideration.
As described above, a satisfactory high-dielectric constant ceramic composite has not been obtained, which exhibits a low temperature coefficient of the dielectric constant and a small change in its properties under a heat-cycle test and a high-humidity load test.